Fuel cell power plants are used increasingly to provide electrical power for a variety of end uses. The fuel cell power plant typically includes one or more fuel cell stack assemblies (CSA), each consisting of an anode, a cathode, and an electrolyte that separates the anode and cathode. Fuel reactant (for a PEM fuel cell), which is typically a hydrogen-rich stream, enters the anode of the CSA, and an oxidant reactant, typically air, enters the cathode. A catalyst in the anode causes the hydrogen to oxidize, resulting in the creation of hydrogen ions, which pass through the electrolyte to the cathode to create the electrical current of the power plant. In addition to the CSA, the power plant typically also includes a fuel processing system (FPS) for converting a hydrocarbon feedstock, such as natural gas, LPG, gasoline, and/or numerous others, to the hydrogen-rich fuel stream, which fuel stream may be referred to as “reformate” because of an included process of reformation.
Although the types of fuel cells vary according to their electrolytes, they may also be viewed as varying according to their operating temperatures. One type of fuel cell that is receiving considerable attention for application to automotive and other uses is the PEM fuel cell that employs a solid polymer electrolyte referred to as a proton exchange membrane. These fuel cells typically operate at temperatures of 60° C. to 83° C., though temperatures to less than 38° C. and as great as 120° C. are possible, and within a pressure range of about one to five atmospheres. Moreover, the catalyst associated with the anode of a PEM fuel cell is particularly susceptible to being “poisoned” by carbon monoxide (CO) that may be contained in the reformate.
Accordingly, the fuel processing system (FPS) of the power plant is not only required to reform a hydrocarbon feedstock to a hydrogen-rich stream to fuel the anode, but it is also required to convert significant levels of CO in the reformate to carbon dioxide (CO2) to thereby reduce the concentration of CO to a level acceptable at the anode. The FPS may similarly be required to remove or convert objectionable sulfur species in the hydrocarbon feedstock in order to avoid damage to the CSA.
In a typical FPS 10 of a fuel cell power plant 15 of the prior art, a portion of which is depicted in FIG. 1, a desulfurizer (not shown) removes organic sulfur components from the fuel, typically at a relatively early stage in the reformation and conversion process. A reformer (not shown) then converts (or reforms) the fuel in a known manner, in the presence of steam (and air), to a reformate mixture 11 of H2, CO, CO2, H2O, (and N2). Thereafter, the desulfurized reformate 11 is supplied to a high-temperature water gas shift reactor (HT WGS) 12 which typically includes a vaporizer 14 and a catalytic reactor 16. Requisite supplies and control of air, steam and/or water to the relevant sections of the FPS 10, though not shown, are implied and well understood. The HT NGS 12 reduces the CO level (i. e. concentration) and enriches the hydrogen level by supplying additional steam or moisture via the vaporizer 14 and reacting it with the reformate 11 in the reactor 16, according to the reaction (and heat of reaction):CO+H2O<=>CO2+H2ΔH1=−41 kJ/mole H2  (1)This reaction is exothermic (in the forward direction) and equilibrium-limited, with lower temperatures favoring higher CO conversions. However, the reaction rate of the HT WGS catalyst increases exponentially with temperature. Thus, the existing practice that optimizes thermodynamics and kinetics of prior existing HT WGS catalysts is to use a second, or low-temperature, water gas shift reactor (LT WGS) 20. The LT WGS 20 typically includes a water vaporizer or cooler (heat exchanger) 22 preceding a catalytic reactor 24. The vaporizer 22 serves as a cooling device and also provides additional steam for the reactor 24. In some architectures the vaporizer may be replaced or assisted by a cooler which will serve to cool the reformate and at the same time use the heat to pre-heat the feed. The catalyst in the LT WGS reactor 24 has typically been Cu/ZnO or the like, or more recently may have been noble metal-based.
Referring further to FIG. 1, the CO level entering the HT WGS 12 may typically be in excess of 100,000 ppmv (parts per million-volume-wet basis) and may be reduced to about 15,000 to 30,000 ppmv by that reactor. The LT WGS 20 further reduces the CO level to about 10,000 to 5,000 ppmv. However, to avoid poisoning the anode 54, it is necessary for the CO level to be below 50 ppmv, and preferably less than 10 ppmv. To further reduce the CO levels to those target levels, the reformate stream from the LT WGS 20 is applied to a preferential oxidation section. This catalytic approach, however, utilizes air to “burn” the CO to CO2, but since H2 is the major constituent in the reformate gas stream (˜50%-30% H2 vs. 1%-0.5% CO), it also burns with CO. These exothermic reactions are shown below, along with the heat of reaction:
 CO+0.5 O2→CO2 ΔH2=−283 kJ/mole CO  (2)H2+0.5 O2→H2O ΔH3=−242 kJ/mole H2  (3)Uniform mixing of the reformate with inlet air assures homogeneous mixing and therefore, effective operation by avoidance of hot spots. To reduce the amount of H2 consumed, this preferential oxidation process has been performed in two, usually adiabatic, stages, as depicted.
The reformate from LT WGS 20 is fed to a high temperature CO preferential oxidizer subsystem (H PROX) 30 (sometimes also referred to as PROX 1), which includes an air mixer 32 followed by a cooler 34 in turn followed by a catalytic reactor 36. An O2/CO ratio slightly above the stoichiometry of reaction (2) is used (in the 0.5-2 regime) and the CO is reduced from the range of 10,000-5,000 ppmv to the range of about 2,000 to 500 ppmv. The reformate from H PROX reactor 36 is then fed to a low temperature CO preferential oxidizer subsystem (L PROX) 40 (sometimes also referred to as PROX 2), which includes an air mixer 42 followed by a cooler 44 in turn followed by a catalytic reactor 46. Here, a significantly higher O2/CO ratio (1.0-4.0, or more) than the stoichiometric ratio for the reaction (2) is used to ensure elimination of CO to concentrations less than 50 ppmv, and typically less than 10 ppmv. Finally, reformate from the L PROX 40 is flowed through an anode precooler 50 and thence to the anode 54 of fuel cell stack assembly (CSA) 56.
Further consideration is given here to various limitations or complexities that arise with the use of the 2-stage PROX section, and particularly the H PROX section, or subsystem, 30. The combined CO selectivity of the two-stage PROX process is about 35%, i.e., for each CO molecule consumed, two H2 molecules are consumed. Thus, there is an efficiency penalty during this process. If it is assumed, for example, that 8,000 ppmv of CO is oxidized in the two-stage PROX system, the overall H2 consumed is roughly 2×8,000 ppmv=16,000 ppmv, or 1.6%, of which up to 75%, i.e. up to 12,000 ppmv, is consumed in the H PROX reactor 36. Moreover, since the CO reaction is highly exothermic and the levels of CO that are consumed are relatively high, the temperature in this adiabatic reactor typically increases more than 100° C. Due to the high reaction exothermicity, it may be prone to overheating, with a concomitant reverse shift reaction that converts CO2 back to CO, or to overcooling, which fails to adequately oxidize the CO. To avoid overheating, it is necessary to carefully regulate the temperature of reformate entering the H-PROX reactor 36, this typically being done via the heat exchanger/cooler 34 in the H PROX section 30. However, as noted, it is equally important to avoid overcooling since the catalyst of the H PROX reactor 36 has a high sensitivity to the inlet temperature of the reformate and to the CO concentration. While the latter is governed by the HT WGS 12 and the LT WGS 20, the former is governed principally by careful regulation of the heat exchanger/cooler 34 in the H PROX section 30. Indeed, if overcooling occurs and the reformate gas stream temperature at the inlet to the H PROX reactor 36 is lower than the catalyst “light-off” temperature, the catalyst will remain inactive, thereby passing unacceptably high levels of CO to the L-PROX section 40 and also creating a “cold” reformate gas stream. Since the L-PROX reactor is designed to operate for an inlet CO level of less than about 2,000 ppmv (wet basis), then the high CO levels may be passed to the anode 54 of the CSA 56 and poison the catalyst there.
A further illustration of the sensitivity of the WGS sections 12, 20 and PROX sections 30, 40 to inlet flow rates, with respect to CO levels and to gas temperatures, is depicted in FIGS. 2 and 3 respectively. The reformate and coolant throughput rates correlate proportionally with system power output. These Figures depict the response of those several stages of the FPS 15 during a power or load step-down, in the absence of a sophisticated control system. More specifically, it is assumed that the associated control system (not shown) adjusts the coolant flow rate in the pre and post H-PROX reactor heat exchangers 34, and 44, simply and directly in accordance with changes of the flow rate of the reformate. During a rapid load step-down of the system from full power (100% to a fraction of that power, e. g. 80%), the reformate flow will be 80% that of full power and therefore, the flow rate of these heat exchangers is 80% that of full power. Thus, the depictions of FIGS. 2 and 3 illustrate that while the system is operating at full power steady state, the desired reduction in CO may be obtained, yet when the system undergoes a load step-down and may be operating at only 70% or 80%, there is a serious degradation in the CO conversion/removal capability of the two PROX sections 30 and 40. This is because of overcooling, and demonstrates the need for a sophisticated coolant control system to avoid these characteristics and their adverse consequences.
A similar behavior is depicted in FIG. 4, which illustrates a sharp increase in the CO level at the output of L PROX reactor 46 when the power plant 10 experiences a 30% step-down in power (from 100% to 70%) and the control of the coolant system provides only a proportional 30% reduction in the coolant flow rate in the heat exchangers 32, 44. This creates an “overcooled” condition that adversely affects the reaction in the H PROX 36, and thus also the resultant CO level in the reformate issuing from the L PROX 46 downstream thereof.
Further, this high exothermicity of the H PROX reactor 36 can lead to unstable operation states referred to as “bifurcations”, in which the system may be seen to operate in multiple “steady states”. This is due to the coupling effects in the changes of the O2/CO ratio and the coolant flow rates during operation of the system in a transient mode, and leads to either serious overheating, i. e., temperature runaway, or to overcooling, i. e., process extinguishing. As in the examples described above, these modes may be mitigated only with the use of sophisticated, and therefore expensive, coolant and air control systems.
In view of the foregoing discussion of the operating dynamics of the various reaction sections presently used, the further requirement for sophisticated and costly coolant flow control and air control will be understood and appreciated. Such need, or burden, is particularly manifested in the operation of the 2-stage PROX section in the FPS 15, which imposes certain burdens on the power plant 10, to wit, the cost, weight and volume of the PROX hardware itself as well as the extra cost of the sophisticated coolant flow and air controls required to avoid the limitations of a simpler proportional control system.
Accordingly, it is an object of the invention to provide an improved fuel processing system for a fuel cell power plant.
It is a further object of the invention to provide a fuel processing system that requires relatively less equipment.
It is a still further object of the invention to provide a fuel processing system that does not require relatively sophisticated/costly associated controls for thermal and/or air management.
It is an even further object of the invention to provide an efficient, smaller size and weight (compact) and cost effective fuel processing system for providing reformate with an acceptably low CO concentration, to a fuel cell stack assembly.